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Appendix  VL                          1171

ing the wheel at 1400 ft. per sec., expanding in the nozzle 1*63
times. In the reaction turbine there must be at least five wheels
if a pressure of 100 Ibs. is to be decreased to 15 Ibs. absolute (see
p. 894), and the passages should be more contracted, thus, in
combination with slower speed, compelling the delivery of pres-
sure energy. While Parsons adopted the pressure form to
decrease his revolutions, de Laval accepted the speed, reducing it
afterwards; and so the turbine has been developed on widely
different lines.

Steam Turbine Types may be represented in classes:
Impulse Turbines.—The de Laval turbine is illustrated in Fig.
1031. Steam at from 50 to 200 Ibs. pressure enters through the
stop valve A, the gauze strainer B and the governor throttle D
into the annular chamber E, reaching the wheel by three or four
nozzles and exhausting at L, The nozzle s (enlarged section)
is set at 20° to the wheel, and at ^-" clearance from it. It
gradually increases in internal diameter to give the 1-63 to i
expansion, and is opened or closed by the wheel w and screwed
spindle v* The steam reaches the wheel with efflux velocity of
4127 ft. per sec., and the blade centres should travel at 47 % of
this or 1940 ft. per sec. if a theoretical efficiency of 88% be
desired, the leaving velocity being then 34% of the initial, or
1360 ft per sec. By theory this should give n H.P. per Ib. wt.,
or 9 Ibs. per H.P. hour. In practice the peripheral velocity is
reduced to 1380 ft. per sec., thus reversing the conditions of
entering and leaving, i and the pressure decreases from 215 Ibs. to
•93 Ib. absolute into a vacuum of 28". The wheel is mounted
on a thin flexible shaft G supported at its ends in self-adjusting
bearings, j being spherical and H having floating discs. On
rotation the smallest lack of balance causes the wheel centre to
move outward until the centrifugal force is sufficiently resisted by
the shaft stiffness, when a-1 critical velocity' of stability is secured
and vibration ceases. This (settling down? occurs at |th to ^th of
the maximum speed, and is given in revolutions as


^r     •

where 8 is the static deflection due to disc weight.   Experimenters